Apex domains cannot have CNAME records.
Question and response
Question
dig u-strasbg.fr. NS
Answer
u-strasbg.fr. 5 NS shiva.jussieu.fr.
u-strasbg.fr. 5 NS ns1.u-strasbg.fr.
u-strasbg.fr. 5 NS ns2.u-strasbg.fr.
Authority
The response has no authority records.
Additional
ns1.u-strasbg.fr. 5 A 130.79.200.1
ns2.u-strasbg.fr. 5 A 130.79.200.3
shiva.jussieu.fr. 5 A 134.157.0.129
ns1.u-strasbg.fr. 5 AAAA 2001:660:2402:0:0:0:0:1
ns2.u-strasbg.fr. 5 AAAA 2001:660:2402:0:0:0:0:3
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0